Induction-heating apparatus

ABSTRACT

An excitation coil and/or a core member, which has a characteristic frequency other than frequencies used, is employed. Thereby, resonance is prevented between adjacent coils or between a coil and an adjacent component such as a core member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heating device that produces heat bymaking use of induction heating, and a fixing unit in which the heatingdevice is mounted.

2. Description of the Related Art

A heating device using induction heating is employed in a fixing devicethat is mounted in an electrophotographic copying machine.

As is disclosed in, for instance, Jpn. Pat. Appln. KOKAI Publication No.9-258586, in this kind of heating method, eddy current is caused in afixing (heating) roller, using a coil that is wound around a coreextending along the rotational axis of the roller. Thus, the roller isheated.

Jpn. Pat. Appln. KOKAI Publication No. 8-76620 discloses a heatingdevice wherein magnetic field generating means applies a magnetic fieldto a heating belt so that the heating belt produces heat by inductionheating. The heating belt is clamped between a pressing belt and thefield generating means, thus forming a nip.

In this type of heating device using induction heating, radio frequency(RF) power is supplied to the excitation coil, thereby to quickly raisethe temperature up to a level that is needed for fixation. As a result,resonance noise is produced due to resonance of the excitation coil.

Consequently, there arises such a problem that a holder member thatholds the excitation coil, or a coil unit that includes a magnetic corefor enhancing magnetic flux may be damaged.

BRIEF SUMMARY OF THE INVENTION

The present invention can provide a heating device using inductionheating, which can prevent resonance of an excitation coil and canprevent damage to other device components disposed near the excitationcoil.

According to an aspect of the present invention, there is provided aheating device comprising: a coil with a predetermined characteristicfrequency; a control section that supplies power with a predeterminedfrequency to the coil; and an electrically conductive member thatproduces heat by a magnetic field that is generated by the coil, whichis supplied with predetermined power from the control section, whereinthe predetermined characteristic frequency of the coil differs from arange of frequencies of voltage and current that are output from thecontrol section.

According to another aspect of the present invention, there is provideda heating device comprising: a first coil that has a first inductanceand is supplied with power having a first frequency; a second coil thathas a second inductance and is supplied with power having a secondfrequency; a control section that supplies predetermined powers to thefirst and second coils at a predetermined timing; and an electricallyconductive member that produces heat by a magnetic field that isgenerated by the first and second coils, which are supplied with thepredetermined powers from the control section, wherein the controlsection supplies power of the first frequency to the first coil, andpower of the second frequency to the second coil.

According to further another aspect of the present invention, there isprovided a heating device comprising: a coil that is supplied withpredetermined power and generates a predetermined magnetic field; a coremember with a predetermined characteristic frequency, the core memberbeing disposed near the coil; a control section that supplies power witha predetermined frequency to the coil; and an electrically conductivemember that produces heat by a magnetic field that is generated by thecoil, which is supplied with the predetermined power from the controlsection, wherein the predetermined characteristic frequency of the coildiffers from a range of frequencies of voltage and current that areoutput from the control section.

Additional objects and advantages of an aspect of the invention will beset forth in the description which follows, and in part will be obviousfrom the description, or may be learned by practice of the invention.The objects and advantages of an aspect of the invention may be realizedand obtained by means of the instrumentalities and combinationsparticularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of an aspect of the invention.

FIG. 1 schematically shows an example of a fixing unit in which aheating device according to the present invention is disposed;

FIG. 2 schematically shows an example of a heating device that isapplicable to the fixing unit shown in FIG. 1;

FIG. 3 schematically shows an example of excitation coils that areprovided in the heating device shown in FIG. 2;

FIG. 4 schematically shows an example of arrangement of excitation coilsin the heating device shown in FIG. 2;

FIG. 5 is a cross-sectional view of the heating device shown in FIG. 2;

FIG. 6 is a block diagram for illustrating a control system for theheating device shown in FIG. 2;

FIG. 7A and FIG. 7B are schematic cross-sectional views of the heatingdevice shown in FIG. 2;

FIG. 8 is a schematic view for illustrating an example of the method ofmeasuring a characteristic frequency; and

FIG. 9, FIG. 10 and FIG. 11 show examples of a core member in thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

An example of a fixing unit according to an embodiment of the presentinvention will now be described with reference to the accompanyingdrawings.

As is shown in FIG. 1, a fixing unit 1 includes a fixing (heating)roller 2, a press roller 3, an abnormal heating sensor 7, a temperaturesensor 9, magnetic field generating means 10 and an insulation sheet 11.

The heating roller 2 includes an electrically conductive member 2 a thathas a hollow cylindrical shape and is formed of a metal. The conductivemember 2 a has a thickness of about 0.5 to 3.0 mm, preferably about 1.5mm. It is preferable that the outside diameter of the conductive member2 a be φ=60 mm. In this embodiment, the heating roller 2 is made of ion.Alternatively, the heating roller 2 may be formed of, for instance,stainless steel, nickel, aluminum, or an alloy of stainless steel andaluminum. The surface of the conductive member 2 a is provided with areleasing layer 2 b that has a predetermined thickness and is formed ofa fluoro-resin, typically tetrafluoroethylene (TFE) or Teflone.

The press roller 3 includes a metal core 3 a, which is a metallic shaftwith a high rigidity or a rigidity that does not permit deformationunder predetermined pressure; silicone rubber 3 b provided around themetal core 3 a; and fluoro-rubber 3 c. It is preferable that the outsidediameter of the press roller be φ=60 mm.

The press roller 3 receives an urging force from a pressing mechanism(not shown), thereby applying a predetermined pressure to the heatingroller 2. By this pressure, a nip 4 is formed. The nip 4 has apredetermined nip width in a direction perpendicular to the axis of thepress roller 3.

The heating roller 2 is rotated in the direction of an arrow (CW) by adriving motor (not shown). With this rotation, the press roller 3 isrotated in the direction of an arrow (CCW).

The abnormal heating sensor 7 comprises thermostats, for instance. Thesensor 7 detects abnormal heating when the surface temperature of theheating roller 2 rises abnormally. In case abnormal heating occurs,power supply to the magnetic field generating means 10 (excitationcoils), which is described later, is stopped. As will be described laterwith reference to FIG. 6, the abnormal heating sensor 7 comprises atemperature detection element 7 a that is disposed substantially at amidpoint in the longitudinal direction of the roller 2, and atemperature detection element 7 b that is disposed at one end in thelongitudinal direction of the roller 2. A plurality of sensors 7, e.g.two sensors 7, may be provided.

The temperature sensor 9 comprises thermistors, for instance. The sensor9 detects the temperature of the outer periphery of the heating roller2. The temperature sensor 9 comprises a temperature detection element 9a that 7 is disposed substantially at a midpoint in the longitudinaldirection of the roller 2, and a temperature detection element 9 b thatis disposed at one end in the longitudinal direction of the roller 2. Aplurality of temperature sensors 9, e.g. two sensors 9, may be provided.

The order of arrangement and the positions of the abnormal heatingsensor 7 a, 7 b and temperature sensor 9 a, 9 b are not limited to thoseshown in FIG. 1.

The magnetic field generating means 10 is disposed within the heatingroller 2.

The insulation sheet 11 is disposed between the heating roller 2 and themagnetic field generating means 10. The insulation sheet 11 effectsinsulation between the inner peripheral surface of the heating roller 2and the magnetic field generating means 10.

The insulation sheet 11 needs to have a heat-resistance temperature thatis higher than a highest temperature of the heating roller 2, which isheated by induction heating when predetermined power is fed to themagnetic field generating means 10. In addition, the insulation sheet 11needs to have a power resistance that can withstand a maximum power(voltage and current), which is supplied to the magnetic fieldgenerating means 10. Taking these requirements into account, it ispreferable that the insulation sheet 11 have a contraction ratio of 2%or less and a thickness of 0.4 mm or more under the condition in whichthe temperature of the heating roller 2 takes a highest value.

In the present embodiment, the insulation sheet 11, which meets theabove requirements, is formed of PFA (perfluoroalkoxy alkan).Alternatively, PTFE (polytetrafluoroethylene), etc. may be used if theabove conditions of heat-resistance temperature and power resistance aresatisfied.

FIG. 2 is an exploded perspective view that schematically shows anexample of the structure of the magnetic field generating means 10 inthe state prior to assembly.

The magnetic field generating means 10 includes holders 20 a and 20 b,and coil units 21 a, 21 b and 21 c. The coil unit 21 a includes a coremember 22 a, a coil bobbin 23 a and an excitation coil 24 a. The coilunit 21 b includes a core member 22 b, a coil bobbin 23 b and anexcitation coil 24 b. The coil unit 21 c includes a core member 22 c, acoil bobbin 23 c and an excitation coil 24 c.

The holders 20 a and 20 b vertically sandwich the coil units 21 a, 21 band 21 c and hold them in proper positions. The holders 20 a and 20 bmay be formed of the same components, that is, components that have thesame structure and are formed of the same material.

The coil unit 21 a is disposed at a midpoint in the axial direction ofthe heating roller 2. The coil unit 21 a includes the coil bobbin 23 aand the excitation coil 24 a that is wound around the coil bobbin 23 a.

The coil units 21 b and 21 c are disposed at both sides of the coil unit21 a, that is, at both axial ends of the heating roller 2. The coil unit21 b includes the excitation coil 24 b that is wound around the coilbobbin 23 b, and the coil unit 21 c includes the excitation coil 24 cthat is wound around the coil bobbin 23 c.

The core members 22 a, 22 b and 22 c have rectangular shapes withpredetermined sizes, and are disposed inside the coil bobbins 23 a, 23 band 23 c, respectively. In the present embodiment, the core members areformed of ferrite or laminated steel plates. Alternatively, they may beformed essentially of, e.g. dust cores with low loss in radio-frequencyranges.

Preferably, the holders 20 a and 20 b and coil bobbins 23 a, 23 b and 23c should be formed of, e.g. a resin material with high heat resistanceand high insulation properties. Examples of the material of the holders20 a and 20 b and coil bobbins 23 a, 23 b and 23 c include liquidcrystal polymers, engineering plastics, ceramics, PEEK(polyether-ether-ketone) materials, phenolic materials, and unsaturatedpolyesters.

It is preferable that the excitation coils 24 b and 24 c, as isillustrated in FIG. 3, be formed of a single wire in the same windingdirection in the state in which they are held between the holders 20 aand 20 b. Specifically, it is preferable that the excitation coils 24 band 24 c be disposed such that when the excitation coils 24 b and 24 care connected as shown in FIG. 3 and current is supplied at the sametime to the excitation coils 24 a, 24 b and 24 c, the direction ofcurrent flowing in the excitation coil 24 b becomes equal to that ofcurrent flowing in the excitation coil 24 c, the excitation coils 24 band 24 c being adjacent to each other with respect to an axisperpendicular to the axis of the heating roller 2.

As is shown in FIG. 4, the length of the excitation coil 24 a (centralcoil) is set at L1 so as to be able to heat at least the region (width)of contact between, e.g. an A4-size sheet and the outer peripheralsurface of the roller, when the A4-size sheet is fed with its short sidebeing parallel to the axis of the heating roller 2.

The excitation coils 24 b and 24 c (side-end coils) are regarded as asingle coil, when they are viewed from the aspect of electricalcircuitry. When the excitation coils 24 b and 24 c are aligned with theexcitation coil 24 a, as shown in FIG. 4, it is preferable that alongitudinal-axial length L 2 between the outside ends of the excitationcoils 24 b and 24 c be not less than the length of the short side of anThe excitation coils 24 a, 24 b and 24 c are arranged at intervals ofdistance L3. The distance L3 is defined as a distance that minimizesnon-uniformity in surface temperature of the heating roller 2. Thesurface temperature of the heating roller 2 varies depending on the sizeof to-be-heated matter (sheet) that passes through the nip 4 whileabsorbing a predetermined amount of heat. If the distance L3 is toosmall, the temperature of a surface region of the heating roller 2,which is located between the adjacent coils, becomes higher than thetemperature of the other surface region of the heating roller 2. If thedistance L3 is too large, the temperature of the surface region of theheating roller 2, which is located between the adjacent coils, becomeslower than the temperature of the other surface region of the heatingroller 2. In short, non-uniformity in temperature occurs. In the presentembodiment, the distance L3 is determined, based on actual measurementresults, so as to minimize the non-uniformity in surface temperature ofthe heating roller 2.

Each of the excitation coils 24 a, 24 b and 24 c may be formed of, e.g.litz wire that is composed of a predetermined number of twisted copperwire elements each having an outside diameter of φ=0.5 to 1.0 mm. Thewire elements are coated with insulator such as polyimide. In thepresent embodiment, each coil is designed to be driven with a voltageof, e.g. 100V. For this purpose, litz wire, which is composed of 19copper wire elements each having an outside diameter of φ=0.5 mm, isused.

As will be described later with reference to FIG. 5, each coil issupplied with a voltage and current of a predetermined resonancefrequency, thereby generating a predetermined magnetic field.Consequently, eddy current occurs at predetermined portions of theheating roller 2. Joule heat is produced by the eddy current and theresistance of the heating roller. As a result, the heating roller 2 isheated.

FIG. 5 is a schematic cross-sectional view of the coil unit 21 a, whichis taken along a line perpendicular to the axis of the magnetic fieldgenerating means shown in FIG. 2.

In this embodiment, the excitation coil 24 a is wound, as shown in FIG.5. Specifically, when the excitation coil 24 a is divided into two partson both sides of the core member 22 a, as shown in the cross section ofFIG. 5, the wire of the coil 24 a is wound around the coil bobbin 23 ain a direction perpendicular to the sheet surface of FIG. 5. A firstlayer of winding of the coil unit 21 a comprises seven turns (1 to 7)and a second layer of winding comprises seven turns (8 to 14). In total,the coil unit 21 a comprises 14 turns.

FIG. 6 is a block diagram illustrating an example of a control systemfor the fixing device 1 shown in FIG. 1.

A power supply 31 is connected in series to the thermostats 7 a and 7 b.The power supply 31 is also connected to two inverter drive circuits 33a and 33 b via a rectifier circuit 32.

The inverter drive circuit 33 a is connected to the excitation coil 24a. The inverter drive circuit 33 b is connected to the excitation coils24 b and 24 c. The inverter drive circuits 33 a and 33 b supplypredetermined radio-frequency outputs (current and voltage) to theassociated excitation coils. The inverter drive circuit 33 a includes aswitching element 34 a, a drive circuit 35 a and a thermistor 36 a. Theinverter drive circuit 33 b includes a switching element 34 b, a drivecircuit 35 b and a thermistor 36 b.

Each of the switching elements 34 a and 34 b comprises, for instance, anIGBT (Insulated Gate Bipolar Transistor), and controls an operation ofturning on/off a radio-frequency output (radio-frequency current) thatis to be supplied to the excitation coil 24 a, 24 b, 24 c.

The drive circuits 35 a and 35 b control operations of turning on/offthe IGBTs 34 a and 34 b. Specifically, each drive circuit 35 a, 35 boutputs to the IGBT 34 a, 35 b a control-signal (representative of thenumber of times of switching) for supplying a predetermined output tothe associated excitation coil 24 a, 24 b, 24 c.

The thermistor 36 a, 36 b is disposed near the IGBT 34 a, 34 b andsenses the ambient temperature. A fan 38 may be disposed near the IGBT34 a, 34 b. The IGBT 34 a, 34 b feeds back ambient temperatureinformation that is sensed by the thermistor 36 a, 36 b, therebyinstructing the fan 38 to send air. This prevents the IGBT 34 a, 34 bfrom being excessively heated up to high temperatures.

The inverter drive circuit 33 a is connected to an inverter controlcircuit 37 a, and the inverter drive circuit 33 b is connected to aninverter control circuit 37 b.

The inverter control circuit 37 a, 37 b performs the following driveoperation control. For example, the inverter control circuit 37 a, 37 binstructs production of a radio-frequency output from the IGBT 34 a, 34b. In other words, the inverter control circuit 37 a, 37 b instructs theduration of on-state time of the IGBT 34 a, 34 b, so that each coil 24a, 24 b, 24 c can produce a predetermined heating power output. To bemore specific, the inverter control circuit 37 a, 37 b instructs thenumber of times of turn-on (drive frequency) of the IGBT 34 a, 34 b perunit time. In this embodiment, assume that a radio-frequency power(current and voltage) in a range of 20.05 to 100 kHz is supplied to theexcitation coil 24 a, 24 b, 24 c by using the IGBT 34 a, 34 b, or byvarying the inductance of the excitation coil 24 a, 24 b, 24 c by apredetermined value. The frequencies within this range are used forinduction heating (IH). The frequency of power that is supplied to theexcitation coils is set at 20.05 kHz, in consideration of the technicalrequirements (Radio Law Enforcement Regulations) for approval of typedesignation of new-type copying machines. However, the frequency may beset at 20 kHz or thereabouts.

The thermistors 36 a and 36 b, inverter control circuits 37 a and 37 band fan 38 are connected to an IH control circuit 39. The IH controlcircuit 39 controls the operations of these components.

The IH control circuit 39 includes a CPU 40, a ROM 41 and a RAM 42.

Based on a prescribed program stored in the ROM 41, the CPU 40 performsa control (hereinafter referred to as “induction heating (IH) control”)for causing the excitation coil 24 a, 24 b, 24 c to produce apredetermined heating power, i.e. a coil output. The IH control circuit39 informs the inverter control circuits 37 a and 37 b of a firstfrequency f1 to be supplied to the excitation coil 21 a and a secondfrequency f2 to be supplied to the excitation coils 21 b and 21 c,respectively. It is thus possible to set the magnitude of magneticfield, i.e. heating power, at a desired level, which is output from eachexcitation coil. Based on the heating power, eddy current is generatedin the heating roller 2, thereby to ensure a predetermined image-fixingtemperature (i.e. temperature for fixing a developed toner image onpaper). In general, the numerical value of heating power is managed aspower consumption of each coil. In the description below, it is assumedthat the coil output (power consumption) of each coil is a power that issimply input to the excitation coil.

The RAM 42 can store data necessary for induction heating control.

The IH control circuit 39 may be included in a main control circuit 43that controls the entirety of the fixing device.

The main control circuit 43 is connected to the thermistors 9 a and 9 b.Based on a feedback control, the main control circuit 43 manages the IHcontrol circuit 39 so that the surface temperature of the heating roller2 may be kept uniform in its axial direction.

The power that is supplied from the rectifier circuit 32 to a given one,or all, of the coils may be monitored at all times by detecting thesupplied current and voltage by means of a power detection circuit (notshown). The power detection circuit is provided, for example, betweenthe rectifier circuit 32 and the input terminal of the commercial powersupply 31, or between the rectifier circuit 31 and the inverter drivecircuit 33 a, 33 b. An output from the power detection circuit may bedelivered to the main control circuit 43. Thereby, a result of themonitoring by the power detection circuit is fed back to the invertercontrol circuit 37 a, 37 b at predetermined timing, and abnormality suchas burnout of the inverter drive circuit 33 a, 33 b can be detected.

The surface temperature of the heating roller 2 can be maintained at afixed value in its axial direction by supplying a predetermined power ofa predetermined frequency to the excitation coil 24 a, 24 b, 24 c at apredetermined timing, using control methods that will be describedbelow.

Examples of a control (IH control) for raising the outer peripheralsurface temperature of the heating roller 2 up to a predetermined levelare described.

(First Method)

A first method is described. The temperature detected by the thermistor9 a, which is disposed at a position opposed to the central coil unit 21a, is compared with the temperature detected by the thermistor 9 b,which is disposed at a position opposed to at least one of the end-sidecoil units 21 b and 21 c. Based on the comparison result, apredetermined power is supplied to the central coil or the end-side coilat a predetermined time-duration ratio. In short, in the first method,the coil to be turned on at a predetermined duty ratio is switched in analternate manner. The central and end-side coils, which are suppliedwith predetermined power at predetermined timing, generate magneticfields so as to make the surface temperature of the heating roller 2uniform in its axial direction.

In this case, the width of the end-side coil 24 b, 24 c (i.e. the lengthof end-side coil 24 b, 24 c in the axial direction of heating roller 2),over which wire is wound, is less than that of the central coil 24 a.Thus, there is such a problem that even if the wire is wound around theend-side coil 24 b, 24 c in the same manner with the same number ofturns as shown in FIG. 5, the same performance cannot be obtained.

For example, assume that the central coil 24 a and end-side coils 24 band 24 c are formed with such numbers of turns that these coils have thesame value of inductance (L), which is a characteristics value of coils.In this case, however, the impedance (Z), which is anothercharacteristic value of coils, differs between the coils. Consequently,the impedance of the end-side coil 24 b, 24 c is low. This problem isalleviated by using coil bobbins as shown in FIGS. 7A and 7B.

FIG. 7A shows a central coil unit 21 a, and FIG. 7B shows an end-sidecoil unit 21 b, 21 c.

As is shown in FIGS. 7A and 7B, the length L5 of the coil bobbin 23 b,23 c of the end-side coil unit 21 b, 21 c is made greater than thelength L4 of the coil bobbin 23 of the central coil unit 21 a. Thereby,the distance between the coil 24 b, 24 c of the end-side coil unit 21 b,21 c and the inner peripheral surface of the heating roller 2 isdecreased. Hence, magnetical association between the heating roller 2and excitation coil 24 b, 24 c is enhanced, and the density of magneticflux acting on the heating roller 2 increases. Therefore, theperformance of the end-side coil unit 21 b, 21 c is improved.

(Second Method)

A second method is described. A power to the central coil unit 21 a anda power to the side-end coil unit 21 b, 21 c are supplied at the sametime with equal values or different values. Thereby, predeterminedmagnetic fields are generated so as to make the temperature of theheating roller 2 uniform in its axial direction.

However, if electric powers of the same frequency are supplied at thesame time to the excitation coils 24 a, 24 b and 24 c, adjacent ones ofthem resonate, and a problem of resonance noise arises.

Two methods (2-1) and (2-2) are applicable in order to address thisproblem.

According to the method (2-1), the central coil 24 a and end-side coil24 b, 24 c are formed with such predetermined numbers of turns such thatthe central coil 24 a and end-side coil 4 b, 24 c may have inductance(L) values, a difference between which is relatively large. Thereby,even if the same power is supplied to both coils at the same time, thatis, even if electric powers output from the inverter drive circuits 33 aand 33 b shown in FIG. 6 have the same frequency, a predetermineddifference is present between the frequency of power (i.e. usedfrequency) supplied to the central coil and the frequency of powersupplied to the side-end coil. Therefore, resonance between adjacentcoils can be prevented.

In the method (2-2), the values of electric powers that are supplied tothe central coil 24 a and end-side coil 24 b, 24 c are varied, therebyproviding a predetermined difference between frequencies (usedfrequencies) of powers that are supplied to both coils. Thus, resonancebetween the coils can be prevented. Specifically, the inverter drivecircuits 33 a and 33 b shown in FIG. 6 produce powers with frequencieshaving a predetermined difference.

The values of inductance of both coils in the method (2-1) and thedifference in power to be supplied to both coils in the method (2-2) canbe determined, as desired, within such a range that no resonance occurs,for example, within a range in which a difference of 10 kHz or more isprovided between the frequencies of powers that are to be supplied toboth coils. The range in which no resonance occurs is determined by thecharacteristics of adjacent coils, power supplied to the coils, controlmethods for power supply to coils, etc. This range is defined by actualmeasurement and, needless to say, it is not limited to theabove-mentioned value.

In a case where the coil conductances of the central coil and end-sidecoil are set to be equal, the impedance may be made different.

The above-described IH control methods may be selectively adopted,depending on the operation mode, whereby the heating roller 2 can moreeffectively be heated uniformly in its axial direction.

For example, the first method may be adopted in the case where theheating roller 2 is heated in a state without thermal hysteresis, thatis, when it is heated from normal temperature to a predeterminedtemperature, typically at a time of warming-up (W/U). Thus, the heatingroller 2 can more effectively be heated uniformly in its axialdirection.

The second method is advantageously adopted when the non-uniformity intemperature in the axial direction of the heating roller 2 is to beminimized in the state in which the heating roller 2 is already heatedto a predetermined temperature, typically at a time of an ordinarycopying operation.

Even where the power (current and voltage) with radio frequencies in therange of 20.05 to 100 kHz is used as in the present embodiment, the useof the above-described methods can prevent resonance between adjacentcoils, or between a coil and an adjacent component (e.g. coil bobbin,magnetic core), and can alleviate the problem of resonance noise.

Next, the excitation coils 24 a, 24 b and 24 c are described in greaterdetail.

The excitation coils 24 a, 24 b and 24 c are configured to havecharacteristic frequencies that differ from the range of frequenciesused.

Resonance occurs if the characteristic frequency of the excitation coil24 a, 24 b, 24 c coincides with an integer number of times of the usedfrequency. It is thus desirable that the characteristic frequency of theexcitation coil 24 a, 24 b, 24 c be set at a predetermined frequencythat differs from an integer number of times of each of the frequenciesthat are used most frequently.

In the present embodiment, the frequencies that are used most frequentlyare those used in the warming-up (W/U) operation mode, copy operationmode and ready operation mode, which are about 38 kHz, 30 kHz and 25kHz, respectively. Hence, the characteristic frequencies of theexcitation coils 24 a, 24 b and 24 c are neither frequencies near theused frequencies, 38 kHz, 30 kHz and 25 kHz, nor frequencies near 75kHz, 60 kHz and 50 kHz that correspond to an integer number of times ofthe used frequencies.

Experimental results with the use of the excitation coils 24 a, 24 b and24 c demonstrate that resonance noise (dB) decreased by about 50%,compared to the prior art.

The characteristic frequency of the excitation coil 24 a, 24 b, 24 c canbe measured using measuring equipment, for example, as shown in FIG. 8.

An FFT (fast Fourier transform) analyzer 401 is connected to anacceleration pickup 402 that is coupled to a workpiece, and to anoscillation transmitter 403 that transmits oscillation to the workpiece.

If predetermined oscillation is transmitted from the oscillationtransmitter 403 to the workpiece, the FFT analyzer 401 acquiresinformation on the magnitude of the oscillation, and can measure theoscillation of the workpiece on the basis of a signal from theacceleration pickup 402.

Using this equipment, the characteristic frequency of the excitationcoil 24 a, 24 b, 24 c can properly be set.

In the present embodiment, an Impulse Hammer (manufactured byKabushiki-Kaisha Ono-Sokki Seizo) was used as the oscillationtransmitter.

Even where the power (current and voltage) with radio frequencies in therange of 20.05 to 100 kHz is used as in the present embodiment, it ispossible to prevent resonance between adjacent coils, and the problem ofresonance noise. Therefore, damage to the coil bobbin or core member canbe avoided.

Next, the core member 22 a, 22 b, 22 c are described in greater detail.

The core members 22 a, 22 b and 22 c are configured to havecharacteristic frequencies that are different from the range of usedfrequencies.

It is desirable, as mentioned above, that the characteristic frequencyof the core member 22 a, 22 b, 22 c be set at a predetermined frequencythat differs from an integer number of times of each of the frequenciesthat are used most frequently.

FIG. 9 shows the core member 22 a, 22 b, 22 c with a three-dimensionalrectangular shape. As shown in FIG. 9, the core member has a rectangularbody with rectangular surface having a dimension h1 on one side and adimension r1 on the other side, and a dimension b1 in a directionperpendicular to the rectangular surface.

The characteristic frequency (ω_(n)) of the core member 22 a, 22 b, 22 cis calculated as follows.

The characteristic frequency is expressed by $\begin{matrix}{\omega_{n} = \sqrt{\frac{35k}{17m}}} & (1)\end{matrix}$where $\begin{matrix}{k = {\frac{48 \cdot E \cdot {I1}}{{r1}^{3}} = \frac{4 \cdot E \cdot {b1} \cdot ({h1})^{3}}{{r1}^{3}}}} & (2) \\{{r1} = {\frac{{b1} \cdot ({h1})^{3}}{12}.}} & (3) \\{{The}\quad{core}\quad{mass}\quad m\quad{is}\quad{given}\quad{by}} & \quad \\{m = {\frac{{b1} \cdot {h1} \cdot {r1} \cdot D}{g}.}} & (4)\end{matrix}$

If equations 2, 3 and 4 are substituted, the following equation 5 isobtained: $\begin{matrix}{\omega_{n} = \sqrt{\frac{35 \cdot 4 \cdot g \cdot E \cdot ({h1})^{2}}{17 \cdot D \cdot ({r1})^{4}}}} & (5)\end{matrix}$where

-   -   g (acceleration)=9.8 (m/s2)=9.8×104 (mm/s2),    -   E (core longitudinal elastic coefficient)=1.0 (1.0 to 2.0)×10−4        (Kgf/mm²), and    -   D (core density)=5.0 (g/cm³)=5.0×10−6 (Kg/mm3).

As described above, the core longitudinal elastic coefficient E includesa factor of frequency. Based on equation 5, in order to obtain thecharacteristic frequency of the core member 22 a, 22 b, 22 c in thisembodiment, which excludes the range of used frequencies, f=20.05 to 100(kHz), the core member needs to meet the range of sizes defined by thefollowing formula. $\begin{matrix}\begin{matrix}{\frac{h1}{{r1}^{2}} < 2.7} & \quad & {\frac{h1}{{r1}^{2}} > {6.3.}}\end{matrix} & (6)\end{matrix}$

Thus, if the core member 22 a, 22 b, 22 c is formed to have the shapethat meets formula 6, which is defined based on the used frequencies, itis possible to prevent resonance between adjacent coils, the problem ofresonance noise, and damage to the coil bobbin or core member.

For example, the core member, which has a size of h1=50 mm, r1=24 mm andb1=10 mm, meets the formula 6 since h1/r1 ²=0.086.

In addition, the core member, which has a size of h1=50 mm, r1=28 mm andb1=10 mm, meets the formula 6 since h1/r1 ²=0.063.

In the present invention, the shape of the core member is not limited tothe rectangular shape. Alternatively, the invention is applicable to anE-shaped or T-shaped core member.

FIG. 10 is a cross-sectional view of an E-shaped core member 501, andFIG. 11 is a cross-sectional view of a T-shaped core member 502.

The core member 501, as shown in FIG. 10, comprises three juxtaposedparallel portions and a perpendicular portion that is couples the threeparallel portions in a direction perpendicular to the axis of eachparallel portion. The perpendicular portion has a length b2 and a widthh3. Each parallel portion has a width b3. The sum of the length of eachparallel portion and the width h3 of the perpendicular portion is h2.Each of the parallel portions and perpendicular portion (core member501) has a thickness r2.

In this case, equation 3 is changed to $\begin{matrix}{{r2} = {\frac{{b \cdot h^{3}} - {( {b - {3 \cdot {b1}}} )( {{bh} - {h1}} )^{3}}}{12}.}} & (7)\end{matrix}$

Substituting equations 2, 4 and 7 in equation 1, the characteristicfrequency of the core member 501 is calculated. In order for the thuscalculated characteristic frequency to fall within ranges, which excludethe range of frequencies, f=20.05 to 100 (kHz), used in this embodiment,the core member 501 is formed to have a predetermined size.

Similarly, with respect to the core member 502, equation 3 is changed to$\begin{matrix}{{r3} = {\frac{{b \cdot h^{3}} - {( {b - {b1}} )( {h - {h1}} )^{3}}}{12}.}} & (8)\end{matrix}$

Substituting equations 2, 4 and 8 in equation 1, the characteristicfrequency of the core member 502 is calculated.

The core member 502 comprises a first core portion and a second coreportion that is coupled perpendicular to the first core portion. Thefirst core portion has a length b4 and a width h4. The second coreportion has a width b5. The sum of the length of the second core portionand the width h5 of the first core portion is h4. Each of the first andsecond core portions (core member 502) has a thickness r3.

In order for the thus calculated characteristic frequency to fall withinranges, which exclude the range of frequencies, f=20.05 to 100 (kHz),used in this embodiment, the core member 502 is formed to have apredetermined size.

As has been described above, in the present invention, the excitationcoil and/or core member, which has a characteristic frequency other thanthe used frequencies, is used. Thereby, resonance is prevented betweenadjacent coils or between a coil and an adjacent component such as acore member. Needless to say, the present invention is applicable todevices other than the above-described embodiments. Besides, using theabove-described first and second control methods, resonance can moreeffectively be prevented.

1. A heating device comprising: a coil with a predeterminedcharacteristic frequency; a control section that supplies power with apredetermined frequency to the coil; and an electrically conductivemember that produces heat by a magnetic field that is generated by thecoil, which is supplied with predetermined power from the controlsection, wherein the predetermined characteristic frequency of the coildiffers from a range of frequencies of voltage and current that areoutput from the control section.
 2. The heating device according toclaim 1, wherein the predetermined characteristic frequency of the coildiffers from a frequently used frequency, which is included in a rangeof frequencies of voltage and current that are output from the controlsection.
 3. The heating device according to claim 2, wherein the heatingdevice is mounted in an image forming apparatus that forms an image ofan object to be copied, and said frequently used frequency is afrequency that is used when one of a warming-up operation mode, a readyoperation mode and a copy operation mode is selected.
 4. The heatingdevice according to claim 1, wherein the predetermined characteristicfrequency of the coil differs from a frequency, which corresponds to aninteger number of times of at least one of frequencies of voltage andcurrent that are output from the control section.
 5. The heating deviceaccording to claim 4, wherein the coil includes a first coil that issupplied with power of a first frequency, and a second coil that issupplied with power of a second frequency, and there is a difference of10 kHz or more between the first frequency and the second frequency. 6.The heating device according to claim 4, wherein in a case where thesame power is supplied from the control section to the first and secondcoils, the first coil has a first impedance that is different from asecond impedance of the second coil.
 7. The heating device according toclaim 4, wherein the heating device is mounted in an image formingapparatus that forms an image of an object to be copied, and said atleast one of frequencies is a frequency that is used when one of awarming-up operation mode, a ready operation mode and a copy operationmode is selected.
 8. A heating device comprising: a first coil that hasa first inductance and is supplied with power having a first frequency;a second coil that has a second inductance and is supplied with powerhaving a second frequency; a control section that supplies predeterminedpowers to the first and second coils at a predetermined timing; and anelectrically conductive member that produces heat by a magnetic fieldthat is generated by the first and second coils, which are supplied withthe predetermined powers from the control section, wherein the controlsection supplies power of the first frequency to the first coil, andpower of the second frequency to the second coil.
 9. The heating deviceaccording to claim 8, wherein in a case where the same power is suppliedfrom the control section to the first and second coils, the first coilhas a first inductance that is different from a second inductance of thesecond coil.
 10. The heating device according to claim 9, wherein in acase where different powers are supplied from the control section to thefirst and second coils, the first coil is supplied with power of thefirst frequency, which is different from the second frequency of powerthat is supplied to the second coil.
 11. A heating device comprising: acoil that is supplied with predetermined power and generates apredetermined magnetic field; a core member with a predeterminedcharacteristic frequency, the core member being disposed near the coil;a control section that supplies power with a predetermined frequency tothe coil; and an electrically conductive member that produces heat by amagnetic field that is generated by the coil, which is supplied with thepredetermined power from the control section, wherein the predeterminedcharacteristic frequency of the coil differs from a range of frequenciesof voltage and current that are output from the control section.
 12. Theheating device according to claim 11, wherein the core member is athree-dimensional rectangular body with rectangular surface having adimension r on one side and a dimension h on another side, the shapemeeting the following condition,h/r ²<2.7, or h/r ²>6.3.
 13. The heating device according to claim 12,wherein the core member is formed of a magnetic body.
 14. The heatingdevice according to claim 11, wherein the coil comprises a first coiland a second coil, the first coil being disposed closer to theelectrically conductive member than the second coil.
 15. The heatingdevice according to claim 14, wherein the first coil has a lowerimpedance value than the second coil.
 16. The heating device accordingto claim 15, wherein the first coil and the second coil have an equalinductance value.